Method of preparing non-aqueous electrolyte free of any trace of free aluminum chloride

ABSTRACT

1. A METHOD FOR PREPARING AN ELECTROLYTE FOR USE IN ELECTROCHEMICAL CELLS HAVING A LITHIUM BASE NEGATIVE ELECTRODE AND A METAL CHLORIDE POSITIVE ELECTRODE WHICH IS INSOLUBLE IN METHYLCHLOROFORMATE COMPRISING THE STEPS OF EFFECTING A REACTION BY HEATING OF A SUBSTANTIALLY EQUIMOLECULAR MIXTURE OF POTASSIUM CHLORIDE AND ALUMINUM CHLORIDE AT ABOUT 300*C. TO PRODUCE POTASSIUM TETRACHLOROALUMINATE AS REACTION PRODUCT, UTILIZING A SLIGHT EXCESS OF POTASSIUM CHLORIDE IN SAID MIXTURE TO PREVENT PRESENCE OF ANY FREE ALUMINUM CHLORIDDE IN SAID REACTION PRODUCT, EFFECTING A SOLUTION OF SAID POTASSIUM TETRCHLOROALUMINATE REACTION PRODUCT IN METHYLCHLOROFORMATE AND REACTING SAID SOLUTION IN SUBSTANTIALLY STOICHIROMETRIC RATIO WITH LITHIUM PERCHLORATE TO YEILD LITHIUM   TETRACHLOROALUMINATE DISSOLVED IN SAID METHYLCHLOROFORMATE TOGETHER WITH POTASSIUM PERCHLORATE WHICH IS INSOLUBLE IN SAID METHYCHLOROFORMATE AND DECANTING SAID POTASSIUM PERCHLORATE TO PROVIDE AS AN END PRODUCT SAID ELECTROLYTE OF LITHIUM TETRACHLOROALUMINATE DISSOLVED IN MEHYLCHLOROFORMATE AND FREE OF ANY TRACE OF FREE ALUMINUM CHLORIDE.

0t- 22. 1974 JEAN-PAUL @ABANO ETAL 3,343,414 METHOD OF' PREPAHINGNONAQUEOUS ELECTROLYTE FREE OF ANY TRACE 0F FREE ALUIINUM CHLORIDE IOriginal Filed March 8, 1971 |2-/ l I()j FIG. I

V'OLTS loo 2oo'3oo 40o FIG-2 vnouns United States Patent Oihce 3,843,414Patented Oct. 22, 1974 3,843,414 METHOD F PREPARING NON-AQUEOUS ELEC-TROLYTE FREE OF ANY TRACE OF FREE ALUMINUM CHLORIDE Jean-Paul Gabauo,Poitiers, and Gerard Lehmann, Liguge, France, assignors to Societe desAccumulateurs Fixes et de Traction (Societe Anonyme), Romainville.France Original application Mar. 8, 1971, Ser. No. 121,825, nowabandoned. Divided and this application May 15, 1973, Ser. No. 360,439Claims priority, application France, Mar. 27, 1970, 7011097 Int. Cl.H01m 11/00 U.S. Cl. 136-156 4 Claims ABSTRACT OF THE DISCLOSUREElectrochemical cells embodying a lithium negative electrode, `anon-aqueous electrolyte of lithium tetrachloroaluminate solution inmethylchoroformate, without any trace of free aluminum chloride and apositive electrode comprising a metal chloride (MeCln) insoluble inmethylchloroformate and constituting either a weak acid or a base in theLewis sense. The electrolyte is prepared by reacting a solution ofpotassium tetrachloroaluminate in methylchloroformate with lithiumperchlorate preferably in slight excess and separating insolublepotassium perchlorate resulting from the reaction. The potassiumtetrachloroaluminate itself being prepared by synthesis by heating amixture of potassium chloride and aluminum chloride at about 300 C.using a slight excess of potassium chloride.

This is a division of application Ser. No. 121,825, filed Mar. 8, 1971,now abandoned.

RELATED APPLICATIONS No related applications are co-pending.

BACKGROUND OF lINVENTION This invention essentially relates toimprovements to electrochemical cells with non-aqueous electrolytes.

An electrochemical cell, either primary or secondary, comprises fourfundamental constituents: a positive electrode which is reduced duringdischarge, a negative electrode which is then oxidized, an electrolyteand a separator. When the cell is of the secondary type, the positiveelectrode is oxidized and the negative electrode reduced during charge.

In this unit, it is absolutely necessary that the various constituentsbe compatible. It is not possible to associate any one kind of acti-vematerial with all kinds of electrolyte. Thus, if the active materialreacts with the electrolyte, is modified and decomposes the electrolyte,the association of such an active material with such an electrolyte isnot practicable. This is, for instance, the case of some high energyactive materials such as lithium which is incompatible with aqueoussolutions ordinarily used as electrolytes. That is why electrolytesembodying a suitable non-aqueous solvent are used when they are to beassociated with an active material such as lithium.

Such a cell comprising a negative lithium electrode and an electrolyteconstituted by a suitable solution of lithium tetrachloroaluminate ismethylchloroformate has been disclosed in the U.S. Pat. No. 3,540,938.The active material of the electrode disclosed in that patent wasconstituted by cuprous chloride.

During the tests concerning the behavior of this cell during storage andduring discharges at very slow rates, the electrolyte solvent embodyingmethylchloroformate has been found to decompose, thus evolving gaseousproducts. Such a behavior firstly was detrimental to the opera- Cit tionof electrodes and further precluded the making of sealed cells.

During this study, two causes of such behavior could be found. On theone hand, the presence of aluminum chloride A1Cl3, even -as traces, inthe electrolyte was found to result in the decomposition of the solventconstituted by methylchloroformate ClCOOCHs. On the other hand, it wasalso found that the preparation of an electrolyte free from aluminumchloride AlClz which constituted a necessary requirement alone was notsufcient to prevent the degradation of the electrolyte solvent whichwent on through at a lower rate, when cuprous chloride was used aspositive active material.

Objects and features of the present invention are to remedy thesedrawbacks and to obtain quite stable electrochemical cells in which thedegradation of the solvent of the electrolyte which comprises a lithiumtetrachloroaluminate solution in methylchloroformate is prevented and inwhich this electrolyte is inactive both towards the active material ofthe lithium electrode as towards the positive electrode which comprisesa metal chloride suitably selected for this purpose.

With the invention, it is more especially possible to provide primarycells operating without any secondary reaction resulting in thedecomposition of the electrolyte, which are therefore able to operatewithout gas evolution during storage and during discharge, even atlowrate discharge, and as a consequence can be enclosed with out dangerin sealed containers, even if they are cells of very small sizes, whichare particularly interesting. It is well known that it is difficulttomake sealed cells of very small size since the extent of the secondaryreactions resulting in the gas evolution which prevents the sealing ofthe cells, is proportional to the surface area. This area becomesrelatively more important as the cell volume decreases.

SUMMARY OF INVENTION The present invention relates to electrochemicalcells comprisingv a negative lithium electrode and a nonaqueouselectrolyte comprising a lithium tetrachloroaluminate solution inmethylchloroformate, such cells being more especially characterized inthat the said electrolyte does not contain any trace of free aluminumchloride (AlCl3) and that the positive electrode comprises a metalchloride (MeCln) insoluble in methylchloroformate (ClCOOCH3) andconstituting either a very weak acid or a base of the Lewis type.

The metal chloride satisfying the conditions required for constitutingthe positive electrode may be selected from the group consisting ofsilver chloride, mercurio chloride and in a lesser measure cupricchloride.

When the cell embodies these features the electrolyte is found to bequite stable in its conditions of use. More specially it is evenpossible to make miniature lithum cells of long life and high energydensity, e.g. for watches.

According to another characteristic and vfeature of the' invention, inorder to effect preparation of an electrolyte comprising a lithiumtetrachloroaluminate solution in methylchloroformate which is entirelyclear of free aluminum chloride, the lithium tetrachloroaluminate isprepared by double composition, by making lithium perchlorate react withpotassium tetrachloroaluminate in substantially stoichiometric ratio,the reaction taking place in a methylchloroformate solution. Thereafter,the insoluble potassium perchlorate produced by the reaction isseparated.

Advantageously, a slight excess of lithium perchlorate is used in orderto avoid having free potassium tetrachloroaluminate in the 'nalelectrolyte.

The potassium tetrachloroaluminate itself can be prepared by synthesiscomprising heating a substantially equimolecular mixture of potassiumchloride and aluminum chloride at about 300 C. This synthesis isadvantageously eiected using a slight excess of potassium chloride inorder to prevent having free aluminum chloride remaining after thereaction is completed.

Other objects and features of the invention will become apparent fromthe following detailed description of an ernbodiment and illustrated inthe annexed drawing given solely as example and in which:

FIG. l is a diagrammatic sectional view of a miniature cell embodyingthe invention;

FIG. 2 is a curve showing the voltage of a cell in volts as ordinatesplotted against the time of discharge in hours as abscissae.

DETAILED DESCRIPTION As hereinabove described, a irst purpose of theinvention is obtaining a perfectly stable electrolyte comprisingmethylchloroformate ClCOOC'l-I3 as solvent and lithiumtetrachloroaluminate LiAlCl4 as solute, without any free aluminumchloride A1Cl3 in the electrolyte.

The best presently known method for preparing the lithiumtetrachloroaluminate used as solute, consisted in heating a mixture ofaluminum chloride and lithium chloride at temperatures comprisingbetween approximately 300 C. and 450 C. according to the reaction.

LiCl-l-AlClgLiAlCl Since it is an equilibrium it is never quantitativeand some undesirable free aluminum chloride remains which results in thedecomposition of the methylchloroformate used as solvent during use andoperation of the cell. The subsequent degradation of the electrolyteyields carbon dioxide and methyl chloride regenerating the aluminumchloride which becomes again available for furthering the decompositionprocess.

According to the invention another and novel method is used forpreparing aluminum tetrachloroalurninate. In this novel method,potassium tetrachloroaluminate (KalCl4) is rst prepared by synthesiscomprising heating a substantially equimolecular mixture of potassiumchloride KCl and aluminum chloride AlCl3 at approximately 300 C.according to the reaction.

K010i Li ClOriKAICh-rLAICli l The potassium perchlorate which is quiteinsoluble in methylchloroformate precipitates and may be separated bymerely decanting.

This reaction is advantageously effected with a slight excess of lithiumperchlorate which remains in the nal electrolyte but does not interferewith or affect the operation of the cell. Thus, the presence ofpotassium tetrachloroaluminate in the iinal electrolyte is prevented.

The conversion of potassium tetrachloroaluminate into lithiumtetrachloroaluminate is effected with a concentration of lithiumperchlorate not over l mole per liter since the solubility of lithiumperchlorate in methylchloroformate is about this concentration.

As hereabove mentioned, a second feature or characteristic of theinvention consists in using a positive active material consisting of ametal chloride which does not react with the solvent of the electrolyteconstituted by methylchloroformate and is not substantially dissolved init.

It has been found that aluminum chloride A1Cl3, ferrie chloride FeCl3,cadmium chloride CdCl2 and, in a lesser degree, cuprous chloride CuClare not suitable for constitutin-g a stable positive active materialthat will not react with the electrolyte solvent. It has also been foundthat silver chloride AgCl and mercurio chloride HgCl2 do not react withthe electrolyte solvent and that cupric chloride CuClz causes adecomposition of the said solvent only at a negligible rate.

Some explanations are hereinafter set forth for a better understandingof the complementary aspect of the invention corresponding Ito itssecond feature regarding the search for a positive active material thatis chemically inactive with respect to the electrolyte solvent.

In a general way the Lewis concept concerning base acids is used forreasoning about the chloride ions Cl.

A rst type of reaction defining an acid compound according to Lewis maybe thus represented:

A second type of reaction corresponding to a base according to Lewis maybe represented as follows:

Thus, the solvent constituted by methylchloroformate ClCOO'CHaconstitutes a base in the Lewis sense since it can form the ion C`H3COO+and the ion Clwhen the latter can be lixed by an acceptor of chlorideions C1- which acids in the Lewis sense can do.

It has been found in the researches made for developing lthe. presentinvention that a positive active material made of a metal chloride mustmeet two requirements:

The first requirement is to be practically insoluble in the solvent.Thus, a chloride soluble in the solvent degrades the solvent by a seriesof secondary reactions. That is, for instance, the case for aluminumchloride AlCl3 whose catalytic action in effecting the decomposition ofthe solvent has been mentioned hereinabove.

The second requirement relates to the kind of the metal chloride in thegeneral sense of Lewis. Thus, the more marked the acid character in theLewis sense of the chloride in question is, the higher is the rate ofsolvent attack` Such is the case of aluminum chloride AlCls. Ferricchloride FeCl3 attacks the solvent at a slower rate than aluminumchloride AlCl3 and it is a less strong acid in the Lewis sense. Thus,cadmium chloride CdClZ which is still weaker an acid in the Lewis sensecauses an attack at a slower rate than that of ferrie chloride. Thisrate is very slow for cuprous chloride which has a not very marked acidcharacter according to Lewis, and it is cancelled for silver chloridewhich has a very weak Lewis acid character.

This is the reason why silver chloride is within the scope of thisinvention.

It is easy to understand also the reason why the positive activematerials constituted by mercurio chloride and also by cupric chlorideare part of the present invention due to the fact that the action of theformer on the electrolyte solvent constituted by methylchloroformate isnull whereas it is practically negligible for the latter. Thesecompounds are bases in the Lewis sense.

Sealed miniature cells which may, for instance, be used in watches havebeen made according to the invention.

As shown in FIG. l, the cell C is of the so-called button typecomprising a cup-like casing bottom 10 and a cialsing cover 11, bothmade of stamped nickel plated steel s eet.

Cathode 12 is composed of a mixture of 95% silver chloride (AgCl) and 5%graphite (or any other material conferring a good electricalconductivity to the electrode). This mixture is directly compressed intothe cup-like bottom 10 and held in place as by ring 13 e.g. made ofstamped steel.

Anode 11 is a lithium disk. A nickel plated steel or stainless steelgrid or screen 15 is welded to cover 14 of the cell and is also embeddedin the lithium disk 11. This1 ensures both good cohesion and goodelectrical contact with the cover 14.

A separator 16 comprised of one of several layers of a felt ofcellulosic fibers is interposed between the electrodes 11 and 12. Thisseparator 16 is impregnated with the elec trolyte comprising a solutionof lithium tetrachloroaluminate in methylchloroformate having no tracesof free aluminum chloride and prepared according to the invention ashereinabove described.

An electrical insulating -gasket 17 seals the closure between casingbottom and cover 11 and insulates these two parts from each other. Theseparts 10 and 11 respectively constitute the electrical terminals of thecell.

Cells have been made with the following features:

cathode 12: AgCl 95 %;lgraphite 5% z 800 mg;

theoretical capacity 142 mah.

anode 11:: lithium disk 2 mm. thick with an embedded stainless steelgrid 15 welded to the negative cover 14 electrolyte: CLCOOCH3|1 MLiAlCl4-f-0.1 M LiC104 quantity per cell about 0.15 cm3.

Total volume of the cell C: 0.52 cm.3

Total weight: 1.95 g.

Outer diameter: 11.5 m.

Height: 5 mm.

The electrical characteristics of such a cell were as follows:

open circuit voltage: 3.08 volts.

internal resistance: 150A (decreases to about 50A after a slightdischarge).

average voltage during discharge: 2.60 Volts (with a 8980A lead).

capacity: end voltage 2 v.: 126 mah.

end voltage 2.4 v.: 116 mah.

energy: 325 mwh for a 2 v. end Voltage energy per unit of volume: 620wh./dm.3.

energy per unit of weight: 165 wh./kg.

1. A method for preparing an electrolyte for use in electrochemicalcells having a lithium base negative elecl trode and a metal chloridepositive electrode which is insoluble in methylchloroformate comprisingthe steps of effecting a reaction by heating of a substantiallyequimolecular mixture of potassium chloride and aluminum chloride atabout 300 C. to produce potassium tetrachloroaluminate as reactionproduct, utilizing a slight eX- cess of potassium chloride in saidmixture to prevent presence of any free aluminum chloride in saidreaction product, eiecting a solution of said potassiumtetrachloroaluminate reaction product in methylchloroformate andreacting said solution in substantially stoichirometric ratio withlithium perchlorate to yield lithium tetrachloroaluminate dissolved insaid methylchloroformate together with potassium perchlorate which isinsoluble in said methylchloroformate and decanting said potassiumperchlorate to provide as an end product said electrolyte of lithiumtetrachloroaluminate dissolved in methylchloroformate and free of anytrace of free aluminum chloride.

2. A method for preparing an electrolyte for use in electrochemicalcells having a lithium negative electrode and a metal chloride positiveelectrode comprising reacting a solution of potassiumtetrachloroaluminate in methylchloroformate with lithium perchlorate insubstantially stoichiometric ratio to precipitate insoluble potassiumperchlorate and thereafter separating said insoluble potassiumperchlorate precipitate produced by the reaction.

3. A method for preparing an electrolyte according to claim 2 whereinsaid potassium tetrachloroaluminate is prepared by synthesis comprisingheating a substantially equi-molecular mixture of potassium chloride andaluminum chloride at approximately 300 C., said synthesis being effectedwith use of a slight excess of potassium chloride.

4. A method for preparing an electrolyte according to claim 2, wherein aslight excess of lithium perchlorate is used in precipitating thepotassium perchlorate.

References Cited UNITED STATES PATENTS 3,073,884 1/1963 Pinkertonl36-100 3,540,938 ll/l970 Gabano l36-l55 3,542,602 11/1970 Gabano136-166 ANTHONY SKAPARS, Primary Examiner U.S. C1. X.R.

1. A METHOD FOR PREPARING AN ELECTROLYTE FOR USE IN ELECTROCHEMICALCELLS HAVING A LITHIUM BASE NEGATIVE ELECTRODE AND A METAL CHLORIDEPOSITIVE ELECTRODE WHICH IS INSOLUBLE IN METHYLCHLOROFORMATE COMPRISINGTHE STEPS OF EFFECTING A REACTION BY HEATING OF A SUBSTANTIALLYEQUIMOLECULAR MIXTURE OF POTASSIUM CHLORIDE AND ALUMINUM CHLORIDE ATABOUT 300*C. TO PRODUCE POTASSIUM TETRACHLOROALUMINATE AS REACTIONPRODUCT, UTILIZING A SLIGHT EXCESS OF POTASSIUM CHLORIDE IN SAID MIXTURETO PREVENT PRESENCE OF ANY FREE ALUMINUM CHLORIDDE IN SAID REACTIONPRODUCT, EFFECTING A SOLUTION OF SAID POTASSIUM TETRCHLOROALUMINATEREACTION PRODUCT IN METHYLCHLOROFORMATE AND REACTING SAID SOLUTION INSUBSTANTIALLY STOICHIROMETRIC RATIO WITH LITHIUM PERCHLORATE TO YEILDLITHIUM